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Purpose: For planning CyberKnife stereotactic radiosurgery (CK SRS) of brain metastases (BM), it is essential to precisely determine the exact number and location of BM in MRI. Recent MR studies suggest the superiority of contrast-enhanced 3D fast spin echo SPACE (sampling perfection with application-optimized contrast by using different flip angle evolutions) images over 3D gradient echo (GE) T1-weighted MPRAGE (magnetization-prepared rapid gradient echo) images for detecting small BM. The aim of this study is to test the usability of the SPACE sequence for MRI-based radiation treatment planning and its impact on changing treatment., Methods: For MRI-based radiation treatment planning using 3T MRI in 199 patients with cerebral oligometastases, we compared the detectability of BM in post-gadolinium SPACE images, post-gadolinium MPRAGE images, and post-gadolinium late-phase MPRAGE images., Results: When SPACE images were used for MRI-based radiation treatment planning, 29.8% and 16.9% more BM, respectively, were detected and included in treatment planning than in the post-gadolinium MPRAGE images and the post-gadolinium late-phase MPRAGE images (post-gadolinium MPRAGE imaging: n total  = 681, mean ± SD 3.4 ± 4.2; post-gadolinium SPACE imaging: n total  = 884, mean ± SD 4.4 ± 6.0; post-gadolinium late-phase MPRAGE imaging: n total  = 796, mean ± SD 4.0 ± 5.3; P post-gadolinium SPACE imaging versus post-gadolinium MPRAGE imaging  < 0.0001, P post-gadolinium SPACE imaging versus post-gadolinium late-phase MPRAGE imaging < 0.0001)., Conclusion: For 3T MRI-based treatment planning of stereotactic radiosurgery of BM, we recommend the use of post-gadolinium SPACE imaging rather than post-gadolinium MPRAGE imaging., (© 2022. The Author(s).)